In psychology, personality type refers to the psychological classification of individuals. In contrast to personality traits, the existence of personality types remains extremely controversial. Types are sometimes said to involve qualitative differences between people, whereas traits might be construed as quantitative differences. According to type theories, for example, introverts and extraverts are two fundamentally different categories of people. According to trait theories, introversion and extraversion are part of a continuous dimension, with many people in the middle.

Effective typologies aim to improve understanding of individuals and predict clinically relevant information, as opposed to diminishing knowledge and understanding as occurs in the case of stereotyping. There is an extensive literature on the topic of classifying the various types of human temperament and an equally extensive literature on personality traits or domains. These classification systems attempt to describe normal temperament and personality and emphasize the predominant features of different temperament and personality types; they are largely the province of the discipline of psychology. Personality disorders, on the other hand, reflect the work of psychiatry, a medical specialty, and are disease-oriented. They are classified in the Diagnostic and Statistical Manual (DSM), a product of the American Psychiatric Association.

The term type has not been used consistently in psychology and has become the source of some confusion. Furthermore, because personality test scores usually fall on a bell curve rather than in distinct categories, personality type theories have received considerable criticism among psychometric researchers. One study that directly compared a "type" instrument (the MBTI) to a "trait" instrument (the NEO PI) found that the trait measure was a better predictor of personality disorders. Because of these problems, personality type theories have fallen out of favor in psychology. Most researchers now believe that it is impossible to explain the diversity of human personality with a small number of discrete types. They recommend trait models instead, such as the five-factor model.

One of the more influential ideas originated in the theoretical work of Carl Jung, as published in the book Psychological Types. The original German language edition, Psychologische Typen, was first published by Rascher Verlag [de], Zurich, in 1921. Jung's theory of psychological types is based on the assumption that there are different functions of consciousness and attitudes of consciousness. Typologies such as Socionics, the MBTI assessment, and the Keirsey Temperament Sorter have their roots in Jungian theory.

Jung's interest in typology grew from his desire to reconcile the theories of Sigmund Freud and Alfred Adler, and to define how his own perspective differed from theirs. Jung wrote, "In attempting to answer this question, I came across the problem of types; for it is one's psychological type which from the outset determines and limits a person's judgment." (Jung, [1961] 1989:207) He concluded that Freud's theory was extraverted and Adler's introverted. (Jung, [1921] 1971: par. 91) Jung became convinced that acrimony between the Adlerian and Freudian camps was due to this unrecognized existence of different fundamental psychological attitudes, which led Jung "to conceive the two controversial theories of neurosis as manifestations of a type-antagonism." (Jung, 1966: par. 64)

In the book Jung categorized people into primary types of psychological function.

Jung proposed the existence of two dichotomous pairs of cognitive functions:

Jung went on to suggest that these functions are expressed in either an introverted or extraverted form.